Methane conversion

ABSTRACT

A method for synthesizing hydrocarbons from a methane source which comprises contacting methane with a reducible oxide of Tb. The Tb oxide is preferably combined with an amount of alkali and/or alkaline earth metal which is sufficient to improve the selectivity to higher hydrocarbon products. The oxide is reduced by the contact which is carried at about 500 DEG  to 1000 DEG  C. Reducible oxides of Tb are regenerated by oxidizing the reduced composition with molecular oxygen. The oxide Tb4O7 is particularly effective in the process.

CROSS REFERENCE TO RELATED CASE

This application is a continuation-in-part of U.S. patent applicationSer. No. 06/600,918 filed Apr. 6, 1984, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to synthesis of hydrocarbons from a methanesource. A particular application of this invention is a method forconverting natural gas to a more readily transportable material.

2. Description of the Prior Art

A major source of methane is natural gas. Other sources of methane havebeen considered for fuel supply, e.g., the methane present in coaldeposits or formed during mining operations. Relatively small amounts ofmethane are also produced in various petroleum processes.

The composition of natural gas at the wellhead varies but the majorhydrocarbon present is methane. For example, the methane content ofnatural gas may vary within the range from about 40 to about 95 volumepercent. Other constituents of natural gas include ethane, propane,butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbondioxide, helium and nitrogen.

Natural gas is classified as dry or wet depending upon the amount ofcondensable hydrocarbons contained in it. Condensable hydrocarbonsgenerally comprise C₃ + hydrocarbons although some ethane may beincluded. Gas conditioning is required to alter the composition ofwellhead gas, processing facilities usually being located in or near theproduction fields. Conventional processing of wellhead natural gasyields processed natural gas containing at least a major amount ofmethane.

Large-scale use of natural gas often requires a sophisticated andextensive pipeline system. Liquefaction has also been employed as atransportation means, but processes for liquefying, transporting, andrevaporizing natural gas are complex, energy-intensive and requireextensive safety precautions. Transport of natural gas has been acontinuing problem in the exploitation of natural gas resources. Itwould be extremely valuable to be able to convert methane (e.g., naturalgas) to more readily handleable or transportable products. Moreover,direct conversion to olefins such as ethylene or propylene would beextremely valuable to the chemical industry.

Recently, it has been discovered that methane may be converted to higherhydrocarbons by a process which comprises contacting methane and anoxidative synthesizing agent at synthesizing conditions (e.g., at atemperature selected within the range from about 500° to about 1000°C.). Oxidative synthesizing agents are compositions having as aprincipal component at least one oxide of at least one metal whichcompositions produce C₂ + hydrocarbon products, co-product water, and acomposition comprising a reduced metal oxide when contacted with methaneat synthesizing conditions. Reducible oxides of several metals have beenidentified which are capable of converting methane to higherhydrocarbons. In particular, oxides of manganese, tin, indium,germanium, lead, antimony and bismuth are most useful. Seecommonly-assigned U.S. patent application Ser. Nos. 522,925; 522,944;522,942; 522,905; 522,877; 522,876; and 522,906, all filed Aug. 12,1983.

Commonly-assigned U.S. patent application Ser. No. 522,935, filed Aug.12, 1983, discloses and claims a process which comprises contactingmethane with an oxidative synthesizing agent under elevated pressure(e.g., 2-100 atmospheres) to produce greater amounts of C₃ + hydrocarbonproducts. The entire content of this application is incorporated hereinby reference.

Commonly-assigned U.S. patent application Ser. No. 522,938, filed Aug.12, 1983, discloses and claims a process for the conversion of methaneto higher hydrocarbons which comprises contacting methane with particlescomprising an oxidative synthesizing agent which particles continuouslyrecirculate between two physically separate zones--a methane contactzone and an oxygen contact zone. The entire content of this applicationis incorporated herein by reference.

Commonly-assigned U.S. patent application Ser. No. 522,937, filed Aug.12, 1983, discloses and claims a process for the conversion of methaneto higher hydrocarbons which comprises contacting methane with anoxidative synthesizing agent containing a promoting amount of alkalimetal and/or compounds thereof. The entire content of this applicationis incorporated herein by reference.

Commonly-assigned U.S. patent application Ser. No. 522,936, filed Aug.12, 1983, discloses and claims a process for the conversion of methaneto higher hydrocarbons which comprises contacting methane with anoxidative synthesizing agent containing a promoting amount of alkalineearth metal and/or compounds thereof. The entire content of thisapplication is incorporated herein by reference.

SUMMARY OF THE INVENTION

It has now been found that methane may be converted to higherhydrocarbon products by contacting a methane-containing gas with areducible oxide of terbium.

It has also been found that methane may be converted to higherhydrocarbon products by contacting a methane-containing gas with a solidcomprising: (1) a reducible oxide of terbium and (2) at least one memberof the group consisting of alkali metals, alkaline earth metals, andcompounds thereof. Alkali metals are selected from the group consistingof Li, Na, K, Rb and Cs. Alkaline earth metals are selected from thegroup consisting of Mg, Ca, Sr and Ba.

Methane is desirably contacted with the solid at a temperature withinthe range of about 500° to 1000° C. The atomic ratio of terbium toalkali or alkaline earth metal is desirably within the range of about1-15:1. Hydrocarbons produced by the process may include lower alkanes,lower olefins and aromatics. The terbium oxide is reduced by contactwith methane and is reoxidizable by contact with an oxygen-containinggas.

Incorporating an alkali or alkaline earth metal metal into the contactsolid substantially reduces the formation of combustion products andimproves higher hydrocarbon product selectivity.

DETAILED DESCRIPTION OF THE INVENTION

Reducible oxides of Tb can be supplied from a variety of known sources.The term "reducible" is used to identify those oxides which are reducedby contact with methane at temperatures within the range of about 500°to 1000° C. A preferred oxide is Tb₄ O₇.

The contact solid employed in the present process may contain, inaddition to a reducible oxide of Tb, at least one alkali or alkalineearth metal. Alkali metals are preferred. Sodium and lithium arepresently preferred alkali metals. The amount of alkali/alkaline earthmetal incorporated into the contact solid is not narrowly critical. Thepreferred atomic ratio of the reducible terbium oxide component(expressed as the metal, Tb) to the alkali/alkaline earth metalcomponent (expressed as the metal, e.g., Na) is within the range ofabout 1-15:1, more preferably within the range of about 1-3:1.

The contact solid may also contain other components heretofore referredto as oxidative synthesizing agents. Oxidative synthesizing agentsgenerally comprise at least one oxide of at least one metal, whichoxides when contacted with methane at synthesizing conditions (e.g., ata temperature selected within the range of about 500° to 1000° C.)produce higher hydrocarbon products, co-product water, and a reducedmetal oxide. The composition thus contains at least one reducible oxideof at least one metal. The term "reducible" is used to identify thoseoxides of metals which are reduced by contacting methane at synthesizingconditions (e.g., at temperatures selected within the range of about500°-1000° C.). The term "oxide(s) of metal(s)" includes: (1) one ormore metal oxides (i.e., compounds described by the general formulaM_(x) O_(y) wherein M is a metal and the subscripts _(x) and _(y)designate the relative atomic proportions of metal and oxygen in thecomposition) and/or (2) one or more oxygen-containing metal compounds,provided that such oxides and compounds have the capability ofperforming to produce higher hydrocarbon products as set forth herein.

Oxidative synthesizing agents have previously been found to comprisereducible oxides of metals selected from the group consisting of Mn, Sn,In, Ge, Sb, Pb, and Bi and mixtures thereof. Particularly effectiveoxidative synthesizing agents have been found to comprise a reducibleoxide of manganese and mixtures of a reducible oxide of manganese withother oxidative synthesizing agents.

It is within the scope of the present invention to include othereffective oxidative synthesizing agent components with the combinedterbium oxide/alkali-alkalline earth metal system of the presentinvention. Thus, the terbium oxide/alkali-alkaline earth metal systemmay also contain a reducible oxide selected from the group consisting ofMn, Sn, In, Ge, Sb, Pb, Bi and mixtures thereof.

It is also within the scope of the present invention to include at leastone phosphorus component in the solid contacted with methane.

While the exact composition of the contact solids is more complex, apreferred group of solids employed in the process of this invention maybe described by the following empirical expression:

    Tb.sub.a B.sub.b C.sub.c P.sub.d O.sub.e

wherein B is selected from the group consisting of alkali and alkalineearth metals; C is selected from the group consisting of Mn, Sn, In, Ge,Pb, Sb, Bi and mixtures thereof; a, b, c, d and e indicate the atomicratio of each component; and when a is 10, b is within the range ofabout 0.5-10, c is within the range of about 0-10, d is within the rangeof about 0-10, and e has a value which is determined by the valences andproportions of the other elements present.

These components may be associated with other support materials.However, in a presently preferred embodiment, a reducible oxide of Tb isemployed as a support for the other components of the solids. While useof other supports is within the scope of this invention, it has beenfound that materials such as silica and alumina tend to deactivate theterbium component via the formation of silicates and aluminates.

A particularly preferred embodiment of the present invention comprisescontacting methane at a temperature within the range of about 500° to1000° C. with a solid comprising a member of the group consisting ofalkali metals and compounds thereof associated with a support comprisinga reducible oxide of Tb. Preferably, the reducible oxide of Tb comprisesTb₄ O₇. Still more particularly, the presently preferred alkali metalsassociated with these supports are Na and Li.

The contact solids employed in this invention can be prepared by anysuitable method. Conventional methods such as precipitation,co-precipitation, impregnation, or dry-mixing can be used. Supportedsolids may be prepared by methods such as adsorption, impregnation,precipitation, co-precipitation, and dry-mixing. When phosphorus isincorporated in the agent, it is desirable to provide it in the form ofa phosphate of an alkali metal or an alkaline earth metal. Substantiallyany compound of these elements can be employed in the preparation of thepromoted synthesizing agent.

A suitable method of preparation is to impregnate a support withsolutions of compounds of the desired metals. Suitable compounds usefulfor impregnation include the acetates, acetylacetonates, oxides,carbides, carbonates, hydroxides, formates, oxalates, nitrates,phosphates, sulfates, sulfides, tartrates, fluorides, chlorides,bromides, or iodides. After impregnation the preparation is dried toremove solvent and the dried solid is prepared for use by calcining,preferably in air at a temperature selected within the range of about300° to 1200° C. Particular calcination temperatures will vary dependingupon the particular metal compound or compounds employed.

If phosphorus is used, the alkali/alkaline earth metal and phosphorusare preferably added to the composition as compounds containing both Pand alkali/alkaline earth metals. Examples are the orthophosphates,metaphosphates, and pyrophosphates of alkali/alkaline earth metals.Pyrophosphates have been found to give desirable results. Sodiumpyrophosphate is particularly preferred.

Regardless of how the components of the contact solid are combined, theresulting composite generally will be dried and calcined at elevatedtemperatures prior to use in the process of this invention.

The present process is distinguished from previously suggested methaneconversion processes which rely primarily on interactions betweenmethane and at least one of nickel and the noble metals, such asrhodium, palladium, silver, osmium, iridium, platinum and gold. Anexample of this type of process is disclosed in U.S. Pat. No. 4,205,194.The present process does not require that methane be contacted with oneor more of nickel and such noble metals and compounds thereof.

Moreover, in a preferred embodiment, such contacting is carried out inthe substantial absence of catalytically effective nickel and the noblemetals and compounds thereof to minimize the deleterious catalyticeffects of such metals and compounds thereof. For example, at theconditions, e.g., temperatures, useful for the contacting step of thepresent invention, these metals when contacted with methane tend topromote coke formation, and the metal oxides when contacted with methanetend to promote formation of combustion products (CO_(x)) rather thanthe desired hydrocarbons. The term "catalytically effective" is usedherein to identify that quantity of one or more of nickel and the noblemetals and compounds thereof which when present substantially changesthe distribution of products obtained in the contacting step of thisinvention relative to such contacting in the absence of such metals andcompounds thereof.

In addition to methane, the feedstock employed in the method of thisinvention may contain other hydrocarbon or non-hydrocarbon components,although the methane content should typically be within the range ofabout 40 to 100 volume percent, preferably from about 80 to 100 volumepercent, more preferably from about 90 to 100 volume percent.

Operating temperatures for the contacting of methane-containing gas andthe reducible terbium oxide are generally within the range of about 500°to 1000° C. If reducible oxides of metals such as In, Ge or Bi arepresent in the solid, the particular temperature selected may depend, inpart, on the particular reducible metal oxide(s) employed. Thus,reducible oxides of certain metals may require operating temperaturesbelow the upper part of the recited range to minimize sublimation orvolatilization of the metals (or compounds thereof) during methanecontact. Examples are: (1) reducible oxides of indium, (operatingtemperatures will preferably not exceed about 850° C.); (2) reducibleoxides of germanium (operating temperatures will preferably not exceedabout 850° C.); and (3) reducible oxides of bismuth (operatingtemperatures will preferably not exceed about 850° C.).

Operating pressures for the methane contacting step are not critical tothe presently claimed invention. However, both general system pressureand partial pressure of methane have been found to effect overallresults. Preferred operating pressures are within the range of about 1to 100 atmospheres, more preferably within the range of about 1 to 30atmospheres.

Contacting methane and a reducible terbium oxide to form higherhydrocarbons from methane also produces a reduced metal oxide andco-product water. The exact nature of the reduced metal oxides areunknown, and so are referred to herein as "reduced metal oxides".Regeneration of a reducible metal oxide is readily accomplished bycontacting such reduced materials with oxygen (e.g., anoxygen-containing gas such as air) at elevated temperatures, preferablyat a temperature selected within the range of about 300° to 1200° C.,the particular temperature selected depending on the metal(s) includedin the solid.

In carrying out the present process, a single reactor apparatuscontaining a fixed bed of solids may be used with intermittent or pulsedflow of a first gas comprising methane and a second gas comprisingoxygen (e.g., oxygen, oxygen diluted with an inert gas, or air,preferably air). The methane contacting step and the oxygen contactingstep may also be performed in physically separate zones with solidsrecirculating between the two zones.

Thus, a suitable method for synthesizing hydrocarbons from a methanesource comprises: (a) contacting a gas comprising methane and particlescomprising a reducible Tb oxide to form higher hydrocarbon products,co-product water, and reduced terbium oxide; (b) removing particlescomprising reduced terbium oxide from the first zone and contacting thereduced particles in a second zone with an oxygen-containing gas to formparticles comprising a reducible Tb oxide and (c) returning theparticles produced in the second zone to the first zone. The steps arepreferably repeated at least periodically, and more preferably the stepsare continuous. In the more preferred embodiment solids are continuouslycirculated between at least one methane-contact zone and at least oneoxygen-contact zone.

Particles comprising reducible Tb oxide which are contacted with methanemay be maintained as fluidized, ebullating, or entrained beds of solids.Preferably methane is contacted with a fluidized bed of solids.

Similarly, particles comprising reduced terbium oxide which arecontacted with oxygen may be maintained as fluidized, ebullating orentrained beds of solids. Preferably oxygen is contacted with afluidized bed of solids.

In one more preferred embodiment of the present invention, methanefeedstock and particles comprising a promoted oxidative synthesizingagent are continuously introduced into a methane contact zone maintainedat synthesizing conditions. Synthesizing conditions include thetemperatures and pressures described above. Gaseous reaction productsfrom the methane contact zone (separated from entrained solids) arefurther processed--e.g., they are passed through a fractionating systemwherein the desired hydrocarbon products are separated from unconvertedmethane and combustion products. Unconverted methane may be recoveredand recycled to the methane contact zone.

Particles comprising reduced metal oxide are contacted with oxygen in anoxygen contact zone for a time sufficient to oxidize at least a portionof the reduced oxide to produce a reducible metal oxide and to remove,i.e., combust, at least a portion of any carbonaceous deposit which mayform on the particles in the methane contact zone. The conditions of theoxygen contact zone will preferably include a temperature selectedwithin the range of about 300° to 1200° C., pressures of up to about 30atmospheres, and average particle contact time within the range of about1 to 120 minutes. Sufficient oxygen is preferably provided to oxidizeall reduced metal oxide to produce a reducible oxide and to completelycombust any carbonaceous deposit material deposited on the particles. Atleast a portion of the particles comprising promoted oxidativesynthesizing agent which are produced in the oxygen contact zone arereturned to the methane contact zone.

The rate of solids withdrawal from the methane contact zone is desirablybalanced with the rate of solids passing from the oxygen contact zone tothe methane contact zone so as to maintain a substantially constantinventory of particles in the methane contact zone, thereby enablingsteady state operation of the synthesizing system.

In one alternative process employing the method of this invention, a gascomprising oxygen may be co-fed with a hydrocarbon gas comprisingmethane to the methane contact zone. See U.S. patent application Ser.No. 06/600,656, the entire content of which is incorporated herein byreference.

In a further alternative process employing the method of this invention,the olefin content of the effluent produced by methane conversion asdescribed herein may be oligomerized to produce normally liquid higherhydrocarbon products. See U.S. patent application Ser. No. 06/600,657,the entire content of which is incorporated herein by reference.

In a still further alternative process employing the method of thisinvention, it has been found advantageous to recover C₂ + alkanes from(1) the effuent produced by methane conversion as described hereinand/or (2) streams derived from such effluent and to recycle suchalkanes to the methane contact zone. See U.S. patent application Ser.No. 06/600,878, the entire content of which is incorporated herein byreference.

In a still further alternative process employing the method of thisinvention, it has been found that halogen promoters enhance resultsobtained when methane is converted to higher hydrocarbons by contactwith a reducible metal oxide. See U.S. patent application Ser. No.06/600,668, the entire content of which is incorporated herein byreference. Also see U.S. patent application Ser. Nos. 06/600,659(chalcogen promoters) and 06/600,658 (NO_(x) promoters) the entirecontents of which are incorporated herein by reference.

The invention is further illustrated by reference to the followingexamples.

Methane-contact runs were made at about atmospheric pressure in quartztube reactors (18 mm. inside diameter) packed with 10 ml. of contactsolid. The reactors were brought up to temperature under a flow ofnitrogen which was switched to methane at the start of the run. Unlessotherwise indicated, all methane-contact runs described in the followingexamples had a duration of 2 minutes. At the end of each methane-contactrun, the reactor was flushed with nitrogen and the solids wereregenerated under a flow of air (usually at 800° C. for 30 minutes). Thereactor was then again flushed with nitrogen and the cycle repeated. Mosof the results reported below are based on the cumulative samplecollected after the contact solid has been through at least one cycle ofmethane contact and regeneration.

Experimental results reported below include conversions andselectivities calculated on a carbon mole basis. Carbonate selectivitieswere calculated from the CO₂ content of the N₂ purge gas collected afterthe methane contact run.

Space velocities are reported as gas hourly space velocities (hr.⁻¹) andare identified as "GHSV" in the Examples.

EXAMPLE 1

A contact solid comprising sodium/Tb oxide was prepared by impregnatingTb₄ O₇ with the appropriate amount of sodium (as sodium acetate) fromwater solutions. The impregnated solids were dried at 110° C. for 2hours and then calcined in air by raising the temperature from 200° C.to 800° C. at a rate of about 100° C./hour and then holding thetemperature at 800° C. for 16 hours. The calcined solids contained 4 wt.% Na. Results reported below in Table I are based on analyses ofcumulative samples collected during a two-minute methane-contact run.

                  TABLE I                                                         ______________________________________                                        Temp. (°C.)                                                                             825                                                          GHSV (hr..sup.-1)                                                                              1200                                                         % CH.sub.4 Conv. 30.0                                                         % C.sub.2 + Sel. 43.0                                                         % CO.sub.x Sel.  53.6                                                         % Coke Sel.      0.7                                                          % Carbonate Sel. 2.8                                                          ______________________________________                                    

EXAMPLE 2

The procedure described in Example 1 was repeated except that calcinedTb₄ O₇ was contacted with methane in the absence of sodium. Results areshown in Table II below.

                  TABLE II                                                        ______________________________________                                        Temp. (°C.)                                                                             825                                                          GHSV (hr..sup.-1)                                                                              1200                                                         % CH.sub.4 Conv. 20.0                                                         % C.sub.2 + Sel. 1.8                                                          % CO.sub.x Sel.  94.9                                                         % Coke Sel.      3.2                                                          % Carbonate Sel. 0.1                                                          ______________________________________                                    

EXAMPLE 3

A number of methane contact runs over 4 wt. % Na/Pr₆ O₁₁ (prepared asdescribed in Example 1) were performed to demonstrate the effect oftemperatures and space velocities on process results. Results are shownin Table III below.

                  TABLE III                                                       ______________________________________                                        Temp. (°C.)                                                                         750    750    825   825  850  875                                GHSV (hr..sup.-1)                                                                         1200   3600    600  2400  600  600                                % Conv.     23.7   8.5     44.9 14.3  48.0 53.3                               % C.sub.2 + Sel.                                                                          56.7   71.5    30.3 46.3  18.8 21.8                               % CO.sub.x Sel.                                                                           38.9   22.7    64.5 50.3  80.3 80.8                               % Coke Sel.  0.8   0.8      1.1  0.7   0.4  2.3                               % Carbonate Sel.                                                                           3.7   5.0      4.2  2.8   0.5  6.1                               ______________________________________                                    

What is claimed is:
 1. A method for converting methane to higherhydrocarbon products which comprises contacting at a temperature withinthe range of about 500°-1000° C. a gas comprising methane and a solidcomprising:(a) a reducible oxide of Tb and (b) at least one member ofthe group consisting of alkali metals, alkaline earth metals, andcompounds thereof;provided that when said solid comprises an alkalineearth metal or compound thereof, said contacting is carried out in thesubstantial absence of catalytically effective Ni, Rh, Pd, Ag, Os, Ir,Pt, Au and compounds thereof.
 2. The method of claim 1 wherein the solidis contacted with a gas comprising methane at a temperature within therange of about 700°-900° C.
 3. The method of claim 1 wherein the gascomprising methane contains from about 40 to about 100 volume percentmethane.
 4. The method of claim 1 wherein the gas comprising methanecontains from about 80 to about 100 volume percent methane.
 5. Themethod of claim 1 wherein the gas comprising methane contains from about90 to about 100 volume percent methane.
 6. The method of claim 1 whereinthe gas comprising methane is natural gas.
 7. The method of claim 1wherein the gas comprising methane is processed natural gas.
 8. Themethod of claim 1 wherein component (b) of said solid is selected fromthe group consisting of alkali metals and compounds thereof.
 9. Themethod of claim 8 wherein the alkali metal is selected from the groupconsisting of Li, Na, K, Rb, Cs, and compounds thereof.
 10. The methodof claim 8 wherein the alkali metal is selected from the groupconsisting of sodium, sodium compounds and mixtures thereof.
 11. Themethod of claim 8 wherein the alkali metal is selected from the groupconsisting of potassium, potassium compounds and mixtures thereof. 12.The method of claim 8 wherein the alkali metal is selected from thegroup consisting of lithium, lithium compounds and mixtures thereof. 13.The method of claim 1 wherein the said reducible oxide and thealkali/alkaline earth metal are associated with a support material. 14.The method of claim 13 wherein said reducible oxide is provided as asupport for at least one of the other components of said solid.
 15. Themethod of claim 1 wherein the reducible oxide is Tb₄ O₇.
 16. The methodof claim 1 wherein the atomic ratio of Tb to alkali/alkaline earth metalin said solid is within the ratio of about 1-15:1.
 17. The method ofclaim 1 wherein the atomic ratio of Tb to alkali/alkaline earth metal insaid solid is within the range of about 1-3:1.
 18. The method of claim 1wherein the said solid is described by the empirical formula:

    Tb.sub.a B.sub.b C.sub.c P.sub.d O.sub.e

wherein B is selected from the group consisting of alkali metals,alkaline earth metals, and mixtures thereof; C is a selected from thegroup consisting of Mn, Sn, In, Ge, Pb, Sb, Bi and mixtures thereof; a,b, c, d, and e indicate the atomic ratio of each component; and when ais 10, b is within the range of about 0.5-10, c is within the range ofabout 0-10, d is within the range of about 0-10, and e has a value whichis determined by the valences and proportions of the other elementspresent.
 19. The method of claim 18 wherein the components areassociated with a support material.
 20. The method of claim 19 wherein areducible oxide of Tb is provided as a support for the other componentsof said solid.
 21. The method of claim 20 wherein C is Mn.
 22. Themethod of claim 21 wherein B is Na.
 23. The method of claim 21 wherein Bis K.
 24. The method of claim 21 wherein B is Li.
 25. A method forsynthesizing hydrocarbons from a methane source which comprises:(a)contacting at a temperature within the range of about 500°-1000° C. agas comprising methane and a solid comprising: (1) a reducible oxide ofTb and (2) at least one member of the group consisting of alkali metals,alkaline earth metals, and compounds thereof in an amount sufficient toimprove the selectivity to higher hydrocarbons, said contactingproducing C₂ +hydrocarbons, coproduct water and solids comprising areduced Tb oxide, provided that when said solid comprises an alkalineearth metal or compound thereof, said contacting is carried out in thesubstantial absence of catalytically effective Ni, Rh, Pd, Ag, Os, Ir,Pt, Au and compounds thereof; (b) recovering C₂ +hydrocarbons; (c) atleast periodically contacting the solids comprising reduced Tb oxidewith an oxygen-containing gas to produce a solid comprising a reducibleTb oxide; and (d) contacting a gas comprising methane with the solidsproduced in step (c) as recited in step (a).
 26. The method of claim 25wherein the said solid of step (a) comprises an alkali metal or compoundthereof.
 27. The method of claim 25 wherein the said solid of step (a)comprises an alkali metal or compound thereof on a support consistingessentially of said reducible Tb oxide.
 28. The method of claim 25wherein the temperature of step (c) is within the range of about 300° to1200° C.
 29. A method for converting methane to higher hydrocarbonproducts which comprises contacting at a temperature within the range ofabout 500° to 1000° C. a gas comprising methane and a solid comprising areducible oxide of Tb and at least one member of the group consisting ofalkali metals and compounds thereof.
 30. The method of claim 29 whereinthe alkali metal component of said solid is selected from the groupconsisting of sodium, sodium compounds, and mixtures thereof.
 31. Themethod of claim 29 wherein the alkali metal component of said solid isselected from the group consisting of lithium, lithium compounds, andmixtures thereof.